1. Field of the Invention
The present invention relates to a stabilized hydroxypivalaldehyde (3-hydroxy-2,2-dimethylpropanal, hereinafter referred to as HPA) and a method of stably storing HPA.
2. Description of the Prior Art
HPA is generally produced by aldol condensation of isobutylaldehyde (hereinafter referred to as IBAL) and formaldehyde or a formaldehyde aqueous solution (formalin) in the presence of a basic catalyst such as amines. Although the aldol condensation generally proceeds under either acidic or basic conditions, the synthesis of HPA is generally conducted under basic conditions in the presence of a basic catalyst, because HPA has both a carbonyl group and a hydroxyl group and so easily undergoes self-condensation by acetalization under acidic conditions. After the reaction, the low-boiling components such as unreacted IBAL and formaldehyde are removed by distillation to obtain a reaction production liquid. HPA is generally used as a synthetic intermediate. If neopentyl glycol (hereinafter referred to as NPG), 3-hydroxy-2,2-dimethyl monohydroxypivalate (ester glycol, hereinafter referred to as ESG), etc., which are by-produced during the synthesis of HPA, are intended as the final products, the reaction product liquid of HPA is immediately used in a crude form in the subsequent steps without further purification.
As a method for producing pure HPA, there has been disclosed a method in which a purified HPA is produce by subjecting IBAL and formaldehyde or formalin to aldol condensation in the presence of an amine catalyst to produce HPA, removing low-boiling components by distillation to obtain crude HPA, and purifying the crude HPA by addition of water, crystallization of HPA under cooling, solid-liquid separation of HPA, and washing with water (U.S. Pat. No. 4,036,888 and JP 6-29206B). It has been reported that HPA exists in equilibrium between monomer and dimer (Journal of the Chemical Society, Perkin Transactions II, vol. 3, p. 189–192, 1978) as illustrated below,
and the purified HPA is usually obtained in the form of a dimer. Is has been also disclosed that the HPA dimer show a reactivity equivalent to that of the HPA monomer (JP 1-299239A and JP 5-117187A).
If the crude HPA after removing low-boiling components by distillation is stored under solution conditions containing a basic catalyst, etc., the amount of by-products increases within a short period of time after the distillation to unfavorably reduce the concentration of HPA and/or its dimer. If HPA and/or its dimer is purified by crystallization as proposed in JP 6-29206B, etc., a large amount of HPA and/or its dimer is left in the mother liquor to significantly reduce the yield, this requiring an additional step for recovering HPA and/or its dimer from the mother liquor. Since the wet cake of HPA and/or its dimer is poor in storage stability, the purity can be maintained only in an extremely short period of time. If NPG, ESG, etc. are not the final target compounds and HPA and/or its dimer is not intended to be immediately used as the synthetic intermediate(s), the storage of HPA and/or its dimer which is produced in advance of the use or the transportation for its use at a distant place are extremely limited.
To prevent the purity of HPA and/or its dimer from being lowered, the wet cake obtained in the crystallizing operation for purification should be dried. When heated to 55° C. or higher for drying, HPA and/or its dimer is melted at the initial stage of drying where the water content is still high to increase the escaped amount thereof as the drying proceeds. Simultaneously, as illustrated below, two molecules of HPA react to form ESG by Tishchenko reaction.
To avoid these problems, the drying of the wet cake is effected generally by vacuum dry at temperatures less than 55° C. without heating to high temperatures. However, HPA is lost by sublimation under reduced pressure conditions because of its high vapor pressure and the sublimated HPA causes other problems such as clogging of vacuum lines. Therefore, the wet cake is dried by heating at temperatures lower than 55° C. for a long period of time or dried at temperatures lower than 55° C. in a flow of dry nitrogen, etc. However, these methods make the utility consumption excessively large and are highly disadvantageous for industrial use when taking the need for apparatuses for crystallization, separation, etc. into consideration.
Since HPA is instable under solution conditions containing water or under wet cake conditions, it is generally used in subsequent step without delay after its production. As described above, although it has been proposed to dry the wet cake of HPA, the proposed method is not suitable for industrial use because of the problems mentioned above. Thus, an industrially applicable method of storing HPA for a long period of time without reducing its purity is not hitherto known in the art.